I. Field of the Invention
The present invention relates generally to a chlorination and dechlorination method for the treatment of wastewater or other fluid, or to a chlorination method for the treatment of potable water, and also to an apparatus used in carrying out this method. In particular, the apparatus of the present invention comprises a chlorination unit, a contact tank, and a dechlorination unit for the treatment of wastewater. For potable water treatment, the apparatus comprises a chlorination unit and a contact tank.
II. Background and Description of the Related Art
For years, chlorine has been used extensively as a disinfectant in water and wastewater treatment processes. In fact, chlorine is perhaps the most common water and wastewater disinfectant in use throughout the world today. In large potable water treatment plants or wastewater treatment plants, chlorine gas or liquid is commonly used as a disinfectant. In small plants such as home wastewater treatment plants and in some commercial wastewater treatment plants, chlorine tablets, which are mainly composed of calcium hypochlorite, are used as a disinfectant.
For example, U.S. Pat. No. 5,133,381 to Wood et al. discloses a chemical dispenser for swimming pools which utilizes calcium hypochlorite tablets arranged loosely in a chemical chamber for sanitizing water supplied to a swimming pool. U.S. Pat. Nos. 3,899,425 to Lewis, 4,210,624 to Price, and 5,089,127 to Junker et al. all disclose the use of chlorine tablets for the disinfection of swimming pool water. In these references, however, the tablets are stacked in feeder tubes. Water washes over tablets in the tube, releasing chlorine into the water. U.S. Pat. Nos. 4,584,106 to Held and 3,579,440 to Bradley, Jr. disclose the use of chlorine tablets in conjunction with similar feeder devices for the treatment of hot tub water and sewage water, respectively.
There are four parameters which must be controlled when using tablets in chlorination and dechlorination processes, i.e., (1) chlorine tablet dissolve rate, (2) contact time, (3) flow pattern and (4) dechlorination tablet dissolve rate. Controlling these parameters will result in consistent chlorine residual.
The dissolve rate of chlorine tablets is determined in part by tablet quality and also by the design of the tablet feeder and weir.
Contact time is set as required by the Environmental Protection Agency ("E.P.A.") and local health departments. Because of the importance of contact time, careful attention must be given to the design of the contact chamber so that at least 80 to 90 percent of the wastewater or potable water is retained in this chamber for the specified contact time. The best way to achieve this contact time is by using a plug-flow, otherwise known as laminar flow, type of contact chamber which can be realized by using a series of interconnected basins or compartments.
Thus, the third parameter, flow pattern, can help control contact time. In a plug-flow contact chamber, all fluid within a fluid flow cross section of the flow path is moving at substantially the same flow rate, minimizing short circuiting. Short circuiting occurs when some of the fluid in the flow path moves at a different rate than fluid flowing beside it. Short circuiting results in nonuniform detention times for fluid in the chamber and can be minimized by utilizing a plug-flow design. Such a contact chamber design is disclosed in U.S. Pat. No. 2,955,923 to Atkinson. The Atkinson contact chamber is filled with loose treatment material which dissolves into the water as it passes through the chamber. As previously stated, these types of designs inhibit the development of dead zones with respect to flow that would otherwise reduce the hydraulic detention times. Length-to-width ("L/W") ratios for the contact chamber flow path of at least about 10 to 1 and preferably 40 to 1 will further minimize short circuiting. Short circuiting may also be minimized by reducing the velocity of the wastewater entering the contact tanks.
As with the chlorination tablet dissolve rate, the dissolve rate of the dechlorination tablets is determined in part by tablet quality and also by the design of the tablet feeder and weir.
Based upon the above factors, the L/W ratio and contact detention time are the two most important parameters for a contact tank design. Contact tanks currently on the market for home wastewater treatment plants are deficient in these two areas. The volume of some contact tanks is too small and the detention time is too short. Some of them have L/W ratios that are less than 10 to 1 because they do not utilize plug-flow designs.
Control of the tablet dissolve rate is an important factor in effective wastewater treatment design. Weirs are typically used to control the water line in tablet feeders and supporters, which in turn helps control the tablet dissolve rate. The principle behind weir design is that the flow rate of fluid entering a tablet feeder supporter is proportional to the water depth in the feeder supporter. The contact surface area of the tablets is likewise proportional to the water depth in the feeder supporter. Thus, more calcium hypochlorite is dissolved during high flow rate periods and less calcium hypochlorite is dissolved during low flow rate periods. The feeder supporter weir, which controls the water line and therefore the flow rate, can be designed such that the chlorine residual can be maintained at a consistent level.
The weirs used in some chlorinator products are too wide to obtain the required chlorine residual. These weirs are only suitable for commercial plants, not for home plants. U.S. Pat. No. 4,759,907 to Kawolics et al. discloses a dissolution chamber having chemical tablet feeder devices and adjustable weirs for controlling the flow rate. However, the dimensions of the example chamber disclosed in the reference are such that the L/W ratio is only slightly greater than 2, and the design of the chamber does not allow any flexibility to increase this ratio.
The chlorinator and dechlorinator are separated from the contact tank in most plant designs, which results in more expense when installing a chlorination and dechlorination system, and makes maintenance of the plant difficult. This cost can be greatly reduced and maintenance simplified if the chlorinator, contact tank and dechlorinator are built in one unit, in contrast to the inefficient designs that are prevalent today.
Faced with the foregoing difficulties in the application of a chlorination and dechlorination process, a new chlorination and dechlorination apparatus has been developed to provide an optimum treatment unit which possesses the advantages of a plug-flow pattern, consistent chlorine residual, high efficiency, and easy maintenance.